1. Field of the Invention
This invention relates to a throughhole-filling paste to be filled into throughholes formed in a printed wiring board or a laminate board, and a printed wiring board and a multi-layer printed wiring board using the same.
2. Description of the Related Art
In recent years, a structure forming a via hole just above a throughhole (i.e., Via on PTH) for electrically connecting a conductor layer and the throughhole has been investigated (see Japanese Patent Laid-Open No. 275959/1994). In this structure, it is necessary to deposit copper on a throughhole-filling material filled in the throughhole. It is of importance to obtain a good adhesion of the copper deposit to the throughhole-filling material in order to obtain a high connection reliability.
In cases when a paste for filling throughholes that contains only an inorganic filler such as silica is used, it is required to rough the surface of the throughhole-filling material by subjecting it to a special chemical treatment using chromic acid or potassium permanganate. Thus, it is difficult to reduce production cost.
In order to improve adhesion to a copper deposit, it has been investigated to use a paste for filling throughholes that contains a metal filler such as copper (see Japanese Patent Laid-Open No. 140280/1999).
However, use of a paste for filling throughholes that contains a copper filler can develop a crack-like gap (12) within a throughhole-filling material (100) after curing (see FIG. 1). This crack-like gap is not developed within the throughhole-filling material containing only an inorganic filler, but is liable to develop within a throughhole-filling material containing a metal filler such as copper.
This gap has been considered to be a crack caused by internal stress generated by shrinkage, upon curing, of a resin component of the throughhole-filling material. Accordingly, in order to prevent development of the crack-like gap, it has been studied to add an ingredient that imparts a good flexibility to a cured throughhole-filling material to thereby reduce internal stress (see Japanese Patent Laid-Open No. 311157/1996).
However, it is known that addition of the component which imparts a good flexibility causes a decrease of glass transition point (Tg) of a cured throughhole-filling material, which leads to reduction in heat resistance or increases shrinkage in volume in a solder reflow process (recuring shrinkage) (see Japanese Patent Laid-open No. 199759/1999). Such phenomena as reduction in heat resistance and recuring shrinkage are unfavorable factors which reduce reliability with respect to throughholes formed in a printed wiring board or a multi-layer wiring board.
As has been described hereinbefore, the paste for filling throughholes to which a good flexibility-imparting ingredient has been added has the problems of reduction in heat resistance or formation of dent (sinking or sink mark) on both sides when used as a throughhole-filling material, and hence the paste cannot have been used in the field where a high reliability is required, such as a package for MPU. Thus, it is not easy to obtain a printed wiring board or a multi-layer printed wiring board having a high reliability with respect to throughholes. In addition, pastes for filling throughholes undergo a large fluctuation of viscosity with time. This fluctuation must be depressed in order to obtain stable products. Further, a long shelf life of the products must be secured to ensure stable properties even after storage for a long period of time.
As to shrinkage behavior upon curing of a throughhole-filling paste containing a filler of a base metal such as copper, there is a large difference in curing rate between the central portion and surface portion (portion exposed to outer atmosphere) of a throughhole. To describe specifically, curing rate in the central portion of the throughhole is extremely slower than curing rate in the surface portion of the throughhole.
In consideration of these phenomena, the inventors have surmised that the above-described gap is not a crack due to internal stress as has conventionally been believed, but a so-called blow hole developed by migration of the resin component in the central portion where curing rate is slow to the surface portion where curing rate is fast.
Thus, the inventors have investigated change in curing behavior using various curing agents. As a result, the inventors have found that certain curing agents can control shrinkage behavior of the throughhole-filling paste upon curing to thereby depress development of the gap, thus having completed the invention based on the findings.
Base metals such as copper to be used as a metal filler are known to have a certain kind of catalytic action and exert some influences on the curing reaction of an epoxy resin (see Japanese Laid-Open No. 31307/1997). The problem about shrinkage behavior upon curing seems to be caused by variation of the catalytic effect depending upon oxidation state of the base metal surface. Specifically, anion polymerization is inhibited on a copper surface, whereas anion polymerization is accelerated on an oxidized surface.
In the invention, an imidazolic curing agent having the structure of formula (1) is used. The problem of developing gap to be caused when a throughhole-filling material containing a filler of base metal such as a copper powder can be avoided by using the curing agent of the invention. It is surmised that this imidazolic curing agent functions to adjust fluctuation of the catalytic action due to difference in surface state of the base metal to accelerate uniformity of the shrinkage behavior upon curing. A possible reason therefor is described later.